3. c2c3 Preparing to control the center with d2-d4. 3... c8b6 The idea of this move is to counter d2-d4 with d7-d5, but the knight isn't well-placed on b6, as it closes in the light-squared bishop. 4. d2d4  4... d7d5 The only justification for Black's previous move. 5. e4d5 